1. Field of the Invention
The present invention relates to an electronic component including a piezoelectric thin film, such as a piezoelectric thin-film resonator, and to a manufacturing method for such an electronic component. The present invention also relates to a filter, a duplexer, and an electronic communication apparatus including such an electronic component.
2. Description of the Related Art
The resonant frequency of a piezoelectric resonator utilizing a thickness-extensional-vibration mode of a piezoelectric substrate is inversely proportional to the thickness of the piezoelectric substrate, and therefore, the piezoelectric substrate must be processed to be very thin for use in the ultrahigh frequency range. In practice, however, in terms of decreasing the thickness of the piezoelectric substrate, a few hundred megahertz is the limit of high frequency in the fundamental mode due to limitations of the mechanical strength or the handling conditions, or other factors. In order to overcome this drawback, the following known piezoelectric thin-film resonator having better high-frequency characteristics has been proposed.
In the piezoelectric thin-film resonator shown in FIG. 10, by partially etching a Si substrate 90 by a micro-processing technique, a thin-film support portion 91 having a thickness of a few microns or smaller is formed in a portion of the Si substrate 90, and a ZnO piezoelectric thin film 94 having a pair of excitation electrodes 92 and 93 is disposed on the thin-film support portion 91 (see, for example, Patent Document 1, Japanese Unexamined Patent Application Publication No. 2001-168674, page 3 and FIG. 3). In the piezoelectric thin-film resonator shown in FIG. 10, since the thickness of the thin-film support portion 91 can be decreased by using a micro-processing technique, and the thickness of the ZnO piezoelectric thin film 94 can also be decreased by, for example, sputtering, the frequency characteristics can be increased to a few hundred megahertz or a few thousand megahertz. In this resonator, however, the temperature characteristics of the resonant frequency are decreased because the temperature coefficients of the Young's modulus of both the ZnO piezoelectric thin film 94 and the Si substrate 90 are negative values.
To solve the problem of a decrease in the temperature characteristics of the resonant frequency, a piezoelectric thin-film resonator shown in FIG. 11 has been proposed. In this resonator, a SiO2 thin film is formed on the surface of a Si substrate 100 by, for example, thermal oxidation, and a thin-film support portion 101 is formed by using the SiO2 thin film by partially etching the Si substrate 100. A ZnO piezoelectric thin film 104 having excitation electrodes 102 and 103 on the upper and lower surfaces is disposed on the thin-film support portion 101. In the piezoelectric thin-film resonator shown in FIG. 11, the temperature coefficient of the Young's modulus of the thin-film support portion 101 is a positive value, unlike that of the ZnO piezoelectric thin film 104. Accordingly, by setting the ratio of the thickness of the ZnO piezoelectric thin film 104 to the thickness of the SiO2 thin-film support portion 101 to a suitable value, the temperature characteristics of the resonant frequency can be made stable (see, for example, Patent Document 2, Japanese Unexamined Patent Application Publication No. 58-121817, all pages and all figures). However, in this resonator, the ZnO piezoelectric thin film 104 cannot be symmetrically located with respect to the vibration node of the fundamental thickness extensional vibration. Accordingly, not only the odd-order higher harmonics, such as third and fifth harmonics, but also even-order higher harmonics, disadvantageously generate spurious responses.
A piezoelectric thin-film resonator that can solve this problem is shown in FIG. 12. In this resonator, SiO2 thin films 204 and 205 are symmetrically arranged on a substrate 200 with respect to a ZnO piezoelectric thin film 203 that is provided between electrodes 201 and 202. With this arrangement, the vibration node is positioned at the central portion of the ZnO piezoelectric thin film 203, thereby preventing the generation of spurious responses of even-order higher harmonics (see, for example, Patent Document 3, Japanese Unexamined Patent Application Publication No. 58-137317, all pages and all figures).
In any of the piezoelectric thin-film resonators shown in FIGS. 10 through 12, as shown in FIG. 13, a lower electrode 303 (which is equivalent to the lower electrodes 92, 102, and 201 in FIGS. 10, 11, and 12, respectively) is patterned on a thin-film support portion 302 (which is equivalent to the thin-film support portions 91, 101, and 204 in FIGS. 10, 11, and 12, respectively) formed on a Si substrate 301 (equivalent to the Si substrates 90, 100, and 200 in FIGS. 10, 11, and 12, respectively). On the patterned lower electrode 303, a ZnO piezoelectric thin film (which is equivalent to the piezoelectric thin films 94, 104, and 203 in FIGS. 10, 11, and 12, respectively) is formed. When forming a ZnO piezoelectric thin film by, for example, sputtering, the lower electrode 303 is isolated and becomes electrically floating, thereby having an unstable potential. Accordingly, the surface roughness (Ra) of the ZnO piezoelectric thin film formed on the lower electrode 303 having an unstable potential becomes greater than 10 nm, and the electromechanical coupling coefficient of a resonator produced by using this film becomes 1.5%, resulting in poor piezoelectric characteristics.